A high sensitive and high resolution X-ray imaging apparatus explained above is designed as a system utilizing a photostimulable phosphor, or stimulable phosphor. A basic system of this apparatus has been described in detail in the U.S. Pat. No. 3,859,527. The phosphor used in this system stores a part of energy, upon reception of energy of radiation such as X-ray. This condition is comparatively stable and is therefore maintained for a long period of time. When the phosphor under this condition is irradiated with the first light beam working as the excited (i.e., excitation) light beam, the stored energy is emitted as the second light beam. In this arrangement, as the first light beam, the light having the wavelength in the wider range from the infrared beam to ultraviolet beam can also be used as well as the visible light beam. However, selection depends on the phosphor material used. The second light is also emitted through wide selection from the infrared beam to ultraviolet beam. Selection of such light also depends on the phosphor material used. The second electromagnetic beam is received, converted to an electrical signal by a photoelectric converter, then converted to a digital signal and thereby digital image information can be obtained.
The photostimulable phosphor layer which has been used in the prior art is not transparent for the first light beam, namely the excited light beam, and the second light beam, namely the photostimulable emitted light beam, and showed distinctive scattering phenomenon. Therefore, even when the photostimulable phosphor layer is irradiated with the excited light beam flux in such a size as is equal to one pixel or is smaller than such pixel, the excited light beam flux is scattered very widely and it has also been observed that when the phosphor layer, for example in the thickness of 0.3 mm, is irradiated with the excited light beam flux in diameter of 0.1 mm, the flux is scattered, at the surface opposed to the irradiation surface, up to the size larger than 1 mm in diameter, in some cases, up to the size larger than 3 mm in diameter.
FIG. 1 shows such scattering condition. As a result of such scattering of the light beam flux, if one pixel is sized as 0.1 mm square in this case, a part of the information of the adjacent 100 to 900 pixels is detected as an error when the one pixel is read, thereby the space resolution of image obtained is remarkably deteriorated and the image is naturally defocused. In order to alleviate the scattering of excited light beam, several methods have been proposed. For instance, a method for decomposing white fine particles in the phosphor layer is described in the Japanese Laid-open Patents Nos. 55-146447 and 58-58500, a method for adding a coloring agent which absorbs the excited light beam is described in the Japanese Laid-open Patent No. 61-170740 and a method for forming a coloring agent or white fine particles on a supporting substrate of the photostimulable phosphor is described in the Japanese Laid-open Patent No. 62-211600. These methods have been attempted to improve sharpness of image for an intensifying screen of the X-ray film of the prior art, however, it is obivous that these methods cannot perfectly remove the scattering of the excited light beam. Moreover, the method for forming cracks in the vertical direction in the photostimulable phosphor layer or forming a honycomb structure is disclosed in the Japanese Laid-open Patent No. 60-171500 or it is also attempted that the scattering is prevented by forming a pattern of projected and recessed areas or mosaic patern on the substrate surface. However, these methods cannot also prevent perfectly the scattering of excited light beam, and still provide a possibility of forming Moire patterns on the image obtained.